Method of and apparatus for continuously producing small section welded steel tubes



Jan. 29, 1963 H. BENTELER METHOD OF AND APPARATUS FOR CONTINUOUSLY PRODUCING SMALL SECTION WELDED STEEL TUBES 4 Sheets-Sheet 1 Filed June 12. 1959 INVENTOR WW h as "Q Q Jan. 29, 1963 H. BENTELER 3 75,

METHOD OF AND APPARATUS FOR CONTINUOUSLY PRODUCING SMALL SECTION WELDED STEEL TUBES 4 Sheets-Sheet 2 Filed June 12. 1959 INVENTOR Jan. 29, 1963 BENTELER 3,

METHOD OF AND APP TUS FOR CONTINUOUSLY PRODUCING SMALL SECTION WELDED STEEL TUBES I Filed June 12, 1959 4 Sheets-Sheet 3 m/vewro 1953 H. BENTELER 3, 7 ,4 4

METHOD OF AND APPARATUS FOR CONTINUOUSLY PRODUCING SMALL SECTION WELDED STEEL TUBES Filed June 12, 1959 4 Sheets-Sheet 4 INVENTOR This invention relates to a method of continuously producing small section welded steel tubes and an apparatus for performing the same. 1

As known, small section welded tubes can be continuously produced by first bending a strip into the shape of a slotted tube in a bending mill comprising a plurality of sets of bending rolls, and by welding together the adjacent longitudinal edges of the slotted tube to form a longitudinal weld seam, the resultant continuous tube be-: ing then passed through a cold rolling mill comprising consecutive roll stands with approximately oval grooved shaping rolls placed alternately at relative angles of 90 and driven at progressively higher speeds for reducing the tube to approximately its desired diameter, the final shape being imparted to the tube by a pass through at least one set of circularly grooved shaping rolls. The advantage of such combined roll bending and reducing mills is that they permit tubes of difierent diameter to be produced from strips of the same width in one continuous operation. It is not therefore necessary to use strips of different widths for producing tubes of diiferent final diameters, nor is it necessary, as would otherwise be the case, to replace the several rolls in the bending mill and the welding machine as well as thewelding equipment itself to adapt the plant to a strip of different width. Within the range of diameters of finished tubes that can thus be produced from a strip of the same width such a combined plant permits the larger diameter tubes to be obtained merely by inactivating or dismantling the final roll sets comprised in the rolling mill.

Compared with earlier methods, wherein the diameter of the tube was reduced in a hot rolling reducing mill as it emerged from a bending mill and welding machine, the aforedescribed method of cold rolling the tube to reduce its diameter affords the major advantage of not requiring the tube to be intermediately heated to rolling temperature and of therefore permitting the plant to be continuous in operation, a factor which substantially reduces the production cost of weided steel precision made tubes.

Despite these advantages there is, however, a major drawback. This drawback consists in that the reduction in diameter that can be achieved by cold rolling is much smaller than the reduction obtainable by hot rolling. Whereas, in hot rolling, diameter reductions up to about 70 to 80% of the original cross section can be achieved, the reduction obtainable by cold rolling a welded tube cannot, in order of magnitude, exceed about Another drawback is that this limitation in the possible reduction ratio renders the production of welded tubes of very small diameter, particularly below about 10 mm., an extremely diflicult matter, because the narrowness of the strip that is then required introduces considerable difii culties into the processes of bending the strip into a slotted tube and of welding the same in the welding machine.

Since a very wide range of sizes and diameters of precision-made steel tubes is usually required, the abovedescribed factors force producers either to equip themselves with several plants for producing tubes within different diameter ranges and to operate these plants as their order books may require, or alternatively to make use of plants in which the roll sets can be exchanged for rolling down different strip widths to the desired final diameters, and to accept as unavoidable the drawbacks involved in giving effect to the periodical structural changes that are thus required. These drawbacks are not confined to the waste of time and labor involved in the replacement of rolls in the bending and reducing mills and in the Welding plant. Difficulties also arise in the proper adjustment of the speed ratios for driving the independently driven bending mill in relation to the following reducing mill. Moreover, plants of this kind are naturally extremely complicated and consequently costly to erect,

apart from being more liable to suifer from functional and operational troubles.

. These difiiculties will naturally be all the more evident when it is desired to use such plants for producing small and very small section welded steel precision made tubes,

for instancedown to outside diameters of about 3 mm. and

wall thicknesses of as little as 0.25 mm. for which there is an ever increasing demand more particularly in the automobile, combustion engine, and machine tool in dustries to serve as fuel pipes, hydraulic pipes in brake systems, and oil pipes in lubricating systems. In view of the sensitivity of such small diameter tubes to any kind of buckling stress one of the principal diificulties is th practical impossibility of ensuring that the speed ratios of consecutive roll sets in the independently driven bending mill and in the reducing mill, as well as the speed ratio between bending and reducing mill are accurately matched to avoid subjecting the tube to buckling stresses in any stage of its passage through the production plant.

In view of these difliculties recourse has again been had to earlier methods of producing very small tube sections, tubes of a minimum outside diameter of 10 mm. being first produced in a combined tube shaping and welding machine and these tubes being then drawn down cold in several passes on cold drawing benches, until the desired final diameters have been achieved. Although this method is less liable to introduce troubles, and waste is lower than in the aforedescribed methods, it is nevertheless unsatisfactory both from the technological as well as from the economical angle in view of the known disadvantages the cold drawing process possesses. Apart from the fact that the use of this method foregoes the advantages inherent in a method of continuously producing tubes in one operation in, very great lengths, the limitation imposed upon tube length by the length of the drawing benches necessitating a subdivision of the welded tube into drawing bench lengths, the additional labor involved in the necessity of joining the drawing bench lengths together by weldmg or forging and the loss of material due to the necessary removal of the beaks from the ends of the tubes, as well as the intermediate heat treatment required between consecutive passes through the drawing plates of the drawing bench combine to make this process very expensive.

The present invention is based on the thought that the observance of certain conditions will in fact permit Welded precision-made steel tubes to be produced in a much more economical way by applying the basic principle of production previously hereinabove described, in a manner that overcomes the stated drawbacks.

It is, therefore, the object of the present invention to provide an efficient method of continuously producing small section welded steel tubes, and apparatus for carrying out that method, which method comprises drawing a steel strip through a bending mill, which comprises a plurality of freely rotating undriven bending roll sets, by the traction generated by a cold rolling reducing mill which lacks intermediate circularly grooved roll pairs excess of its yield point, and passing the tube between a welding machine and said cold rolling reducing mill through a plurality of guide rolls which prevent the torsional forces engendered by the cold rolling reducing mill from being transmitted to the section of tube passing through said welding machine, and which at the same time slightly deform the said tube.

The apparatus for carrying out this method comprises an undiiven freely rotating. roll bending mill for bending a strip into a slotted tube, followed by a pair of freely rotating horizontal rolls immediately preceding. a welding machine for closing the slot of the slotted tube, means provided between the closing rolls. and pressure rolls associated with the welding. machine for guiding the edges of the slotted tube and, following said welding machine, pairs of guide rolls for preventing the welded tube from being axially twisted, said guide rolls preceding a cold rolling reducing mill whieh' lacks intermediate circularly gr ooved shaping rolls and comprises only directly consecutive working r'oll sets with closed oval shaping grooves, placed in alternation in planes perpendicular to the axial direction at relative angles of 90 and driven at relative speeds. which exceed the speeds corresponding with the elongation of. the tube due to the rolling action of consecutive roll pairs and. impart a tensile stress to the rolled material in excess of its yield point, the final set of rolls being a pair of circularly grooved shaping rolls. 7

In view of the considerable velocity ratios which the invention proposes to provide between directly consecutive Working roll sets in the cold rolling mill and which not only allow for the elongation of the tube due to the rolling action but at the same time impart an amount of tension to the tube which exceeds its yield point without reaching the ultimate stress, it is possible to produce in one continuous operation welded tubes, more particul arly from cold rolled strip material, and to roll them down to their final outside diameters between about 20 and about 3 mm., with a total reduction in section (both in diameter and wall thickness) of about 80% related to the initial section of the tube. To achieve this result it is essential that a freely entrained bending mill should be used, i.e. one that isnot powered, because this is the only possible way of ensuring that the tube upon entering the cold rolling reducing mill will be completely relieved of all axial thrust and subject only to adequate tension. 7

To achieve the effect envisaged by the invention it is, furthermore, of importance that in the reducing mill the grooves in cooperating rolls of the shaping roll sets should define closed oval contours and that the rolls of consecutive stands should work in rolling planes displaced through 90 angles, no intermediate circular shaping rolls being employed and the oval shaping roll sets being closely consecutive because it is not otherwise possible to impart longitudinal tension in excess of its yield point to the material of the rolled tubes and to achieve that measure of uniformity in the cold flowing deformation which alone will permit the desired considerable reduction in section, amounting to as much as 80%' (both in relation to diameter and possibly also to wall thickmess) to be obtained.

For generating longitudinal tension above the yield point of the tube material, a feature which is primarily responsible for the considerable reduction in section achieved by the rolling mill, the peripheral speeds of the rolls in consecutive working roll stands of the reducing mill must be increased by about /2 to 3% beyond the speeds determined by the elongation of the tube as such, the speed ratios between consecutive roll sets depending upon the number of stands required, in such a manner that said ratio will be less in the case of a larger number of passes than in the case of a smaller number of passes. On the other hand, the number of roll stands will depend upon the overall reduction requiredto propass is a limited figure and may vary, according to the material, between about 5 and 8%, although in exceptional cases it may be higher.

A final important feature of the invention is that the powerful torque applied to the tube by the reducing rolls shall not have any effect upon thetube as it passes through the welding machine. The interposition of sets of guide rolls between the welding machine and the reducing mill ensures that. in tubes of the contemplated small section the otherwise difiicult problem of welding the very thin edges can be satisfactorily solved and even a substantially headless fiush seam produced if certain welding techniques are employed. The tensional stressm-aintained in the tube permits the closing of the edges of the slot-ted'tube to be delayed" until immediately before the tube runs into the welding machine, without any" risk of deformation or'other distort-ionthat might adversely affect the quality ofthe weld seam.

Moreover, separation of the edges of the slot until shortly before the tube enters the welding machine permits the bead, especially inside the tube, to be satisfactorily removedshould' the welding technique employed be one in which the formation of a bead cannot be avoided. In known methods of operation the removal of the bead was an extremely difficult matter in the case of tubes having internal dimensions as small as those herein envisaged, because the thrust imparted to the tube by the tube shaping rolls necessitated thatthe slot edges should be closed a considerable distance ahead of the welding point with the result that in the small available diameters bead removing tools could not be located precisely owing to their considerable length.

The advantages of the method proposed by the present invention over methods hitherto practised arenot confine dto the considerable improvements which continuous production methods afford with regard to economy of production in the case of precision made steel tubes of small and very small gauge. They are also due to the fact that it is. possible in one installation, from a strip of a particular width, to produce tubes of widely diifering sections without the troublesome complication of exchanging the roll sets, as well as to the fact that much less space is needed for accommodating the plant and that the latter is much simpler in construction and therefore less liable to cause trouble in operation.

The considerable reduction in the tube section of up to that can be achieved as a result of the special construction and the particular speed ratios of the roll sets in the proposed cold rolling reducing mill in conjunction With the tension imparted to the rolled material by the drag transmitted to the freely entrained bending mill, provides not only the fundamental conditions necessary for producing tubes of final cross sections that can be varied within very wide limits (for instance from 20 down to about 3 mm.) from a strip of the same given width, merely the last roll stands of the reducing mill being removed or replaced in conventional manner to determine the final gauge of the tube, but the further advantage also arises that the bending of the strip to form a slotted tube as well as the welding operation are performed when the dimensions of the tube are still of an order in which these operations are much easierto control with great precision even when tubes are produced which have diameters as small as for instance, 3 mm.

It will be readily understood that-as is. alsov the case in the earlier method-an exchange of the roll sets comprised in the bending mill, or of the welding machine, or of rolls associated with the welding machine, or of the guide roll sets, is never required.

In principle any of the hitherto conventionally employed welding techniques can also be used in the performance of the method proposed by the present invention, However, in order to avoid the formation especially of a weld bead inside the tube, it is preferable if use is made of an electric welding technique such as the rgon arc welding process. A particularly useful welding technique is high frequency welding which produces a weld head that is far less pronounced than that which arises in resistance butt welding, and which at the same time is capable of providing a good quality seam when the speed of travel of the material is as high as the outstanding performance of the cold rolling reducing mill will permit.

Especially when using the two last mentioned welding techniques and when taking advantage of the considerable reduction ratios of which the method according to the invention is capable, the rate of production of welded and finished tubes can be raisedto an output which is at least times that obtainable by, conventional methods.

It has been found that a useful arrangement is to pass the slotted tube, with its edges separated until shortly before it enters the welding machine, through a pair of rolls disposed in a horizontal plane and embracing the tube from either side for closing the slot, the edges of the slot being guided by a special guide member from the moment they leave the closing rolls to the moment they enter the first pressure roll associated with the welding machine, said guide member being possibly associated with a special tool for the removal of the internal head, if the nature of the welding process should make this desirable.

Although the assembly of freely rotating guide rolls which follows the welding machine, and which consists of pairs of rolls having alternately oval and circular grooves, may be arranged so that consecutive roll pairs work at relative angles of 90, it is preferred not to adopt this latter arrangement because it is then easier to prevent the tube from twisting within the operational range of the welding machine.

Moreover, it is preferred that the rolls of the cold rolling reducing mill which have oval grooves and which it is proposed to arrange so that consecutive pairs of rolls work at right angles, should be so disposed that said roll pairs are alternately inclined in relation to a vertical plane at angles of 45. In the method according to the invention this latter arrangement aifords'the advantage that the weld seam of the tube which is especially sensitive to deformational stress will always be in a zone where the seam is exposed to minimum deformational stress, whilst the tube passes through the relatively crossed consecutive roll stands in which the cross section is alternately squeezed into ovality in two relatively perpendicular directions. This is a matter of especial importance because cold deformation imparts to the raw material of the tube considera le strength coupled with low elongationso that the weld seam and neighboring regions are particularly liable to be affected by any bending stress.

Although the main field of application of the method and apparatus proposed by the present invention is, and its major economic advantages arise in, the production of welded high precision steel tubes having a final outside diameter between and 3 mm., advantages will also be gained in the production of tubes of greater size. For instance, a welded tube of 90 mm. outside diameter and a wall thickness of 1.2 mm. could be reduced to a finished tube of 20 mm. diameter and a w 11 thickness of 1.2

and less, or a welded tube of say 40 mm. outside diameter and 1 mm. wall thickness might be reduced to a tube of any desired final cross section, down to say an outer diameter of 9 mm. and a wall thickness of 1 mm. and less.

Preferably the cold rolling reducing mill will be equipped with two-high roll stands with grooved rolls enclosing an oval contour. However, in certain cases threehigh or even four-high roll stands could be used with roughly oval grooved shaping rolls.

A preferred embodiment of a bending and rolling mill according to the invention will now be described by way 6 of example and with reference to the accompanying drawings, in which FIG. 1 is a schematic in elevational section;

FIG. 2 is a welding machine, likewise in schematic form, shown in a section taken on the line IIII in FIG. 1;

E6. 3 is a plan view of the section of tube in course of being welded;

FIG. 4 is a cross section of the tube in the plane indicated by IV1V in FIG. 3, and

FIG. 5 is a cross section on the line VV in FIG. 1 showing two pairs of rolls of the cold rolling reducing mill working at a relative angle of likewise represented in schematic form.

For producing a welded precision-made steel tube with a final overall diameter of say 4.75 mm. and a wall thickness of 0.7 mm. a cold rolled steel strip 1a, 54 mm. wide and 0.7 mm. thick, is fed into a plant schematically shown in FIG. 1. Preferably the material of the strip is a deep drawing steel of which a section of maximum length is wound on a freely rotatable reel 2. It is withdrawn from this reel exclusively by the tensional pull produced by a cold rolling reducing mill 3 which draws the strip cold through the several forming devices in the direction indicated by arrow x.

A bed 4 to which the several assemblies of the plant are firmly secured carries a circular shearing cutter 5 through which the steel strip 1a is first drawn as it unwinds from the reel 2. The knives on the rollers of the shears trim the rolled raw edges of the cold rolled steel strip 1a to the exact width of say 51 mm. required for forming a finished tube of the above-mentioned dimensions.

The trimmed strip then enters the rolls of a bending mill 6 with roll sets 6a, 6b, 60 which are not driven and freely rotatable, each pair of rolls being located in a plane which is substantially perpendicular to the direction of travel of the strip. The rolls are shaped to bend the strip into a slotted tube of approximately 16 mm. outside diameter. Naturally, both the disposition and shape of the rolls as well as the number of stands can be suitably modified to adapt the arrangement to the given conditions of the bending operation they are required to perform.

Mounted on a more elevated platform 4a of the bed 4, following the bending roll sets in the direction of travel of the strip, is a pair of closing rolls 7 operating in a plane at right angles to the rolls of the bending mill 6. These rolls close the slot of the tube shortly before it enters the welding machine. The pair of rolls 7 is replaceable and rotates freely on vertical shafts which are relatively horizontally adjustable and placed as closely as possible to the following welding machine.

It will be seen more particularly by reference to FIGS. 3 and 4 that a sword 16, adjustably afiixed to a support 9 is located between the pair of closing rolls 7 and pressure rolls 8 associated with the welding machine. The object of this sword 1b is to align the edges of the slot of the tube in known manner with the position of the electrode immediately before the tube passes the welding point.

In the illustrated example a welding machine ill is employed which is one equ'pped for performing a gas-electrical welding operation, more particularly the argon arc process of welding, and to this end it comprises a tungsten electrode 12 supported on a bracket 13 which permits the electrode to be adjusted in the vertical plane. Whereas current is supplied to the electrode through a cable 14 from a welding transformer not specially shown in the drawings, the inert argon gas is admitted through a duct 15 inside the bracket 13 and a nozzle jacket 13a which surrounds the tungsten electrode 12.

The pressure rolls 8, associated with the Welding machine and likewise freely rotatably mounted in substantially horizontal planes, support and guide the welded tube representation of the entire plant in. such a way that the cross section of the slotted tube cannot vary. As will be seen more particularly by reference to FIG. 2 the peripheries of each of the pressure rolls 3 are formed with a groove 8a shaped to conform with the outside diameter of the tube and to surround something over A of the periphery of the tube as it passes under the electrode.

The weld seam produced by the argon arcv welding process is extremely even and flat, and scarcely builds up a head, so that tools for removing the bead and flattening the weld seam are not required either on the outside or on the inside of the seam. Moreover, it has been found that the considerable reduction in diameter during the subsequent cold rolling process levels out minor elevations in the formation of the seam to such an extent that in thefinished tube the presence of the weld'seam is hardly detectable. Nevertheless there would be no objec tion,, for instance in. the eventv of the tube being welded by a resistance. butt welding process, to affixing a suitable bead removing. tool to the sword 1.0 or to, the bracket 9 which carries the sword.

Further in the direction of travel, indicated by the arrow x, of a welded tube 1', immediately behind the weldiingmachine, there is provided an assembly of guide rolls 16 which in the illustrated embodiment consists of four sets of rolls 16a, 16b, 16c and 16d provided alternately with circular and oval grooves. The vertical roll. sets are likewise arranged to be freely rotatable. The purpose of these rolls is to suppress the transmission of torque from the subsequent cold rolling reducing mill 3 to the welded tube 1 and to prevent any such torsional forces from affecting the section of tube passing through the welding machine.

From the guide roll assembly or deforming means 16 the tube 1 passes into the cold rolling reducing mill 3. This is the only assembly of the plant which is positively driven. A drive shaft not shown in the drawings is connected through appropriate transmission means, for instance through V-belt transmissions 17, with infinitely variable gearings, likewise not specially shown, which are associated with several roll stands 3a, 3b, 3c and 3d. These permit the speed ratios from roll stand to roll stand to be adjusted in such a way thatthe resultant tensional stress in the welded tube 1 exceeds the yield point of the material and maintanns cold deformational flow across the entire tube section and around its entire periphery.

The tube 1. which is only slightly deformed in the guide roll assembly 16 is rolled down in the cold rolling reducing mill 3 in twenty one passes in the, for instance, 21 roll stands provided for the purposes of the illustrated example, and its diameter is reduced from 16, mm. to 4.75 mm, the weight per meter of the tube being thus reduced from about 0.264 kg. to about 0.07 kg. Consequently the tube emerges from the cold rolling reducing mill at approximately 3.8 times the optimum welding speed which in the argon arc Welding process in its present state of development is about 8 meters per minute, but which can be increased by employing the high frequency welding technique to 40 meters per minute and in the further development of this process to even more.

The working roll sets of the cold rolling reducing mill 3, which are placed in alternation at right angles to one another, and which are provided with oval shaping grooves, roll down the welded tube 1 in consecutive passes to the desired final cross section of the finished tube, precision of the final section being ensured by the provision of a final pair of circular shaping rolls 30! (see FIG. According to the final diameter that is required, a suitable number of roll pairs at the end of the mill can be removed or replaced, the final. circular roll pair 3d always forming the finalstand in the mill. As will be seen by reference to FIG. 5 the working roll stands of the cold rolling reducing mill 3 are placed at an angle of 45 to the vertical and incline in alternation to the right and the left;

Behind the cold rolling reducing. mill 3 is a longitudinally traversible saw 18 which reciprocates at a speed determined by the lengths of tube it is desired to out. A circular blade 18a of the saw 18 cuts the required lengths as it moves in the direction of and at the speed of travel of the issuing tube, the cut lengths being discharged on to a conveyor frame 19 or into. a basket whence they are removed for use as required.

According to the purpose for which the tubes are intended, the cut lengths maybe annealed in known manner before further processing.

While the method herein described, and the form of apparatus for carrying this method into effect, constitute preferred embodiments of the invention, it is to 'be understood that the invention is not limited to this precise method and form of apparatus, and that changes may be made in either without departing from the scope of the invention which is defined in the qappended claims.

I claim:

1. Apparatus for continuously producing small section welded tubing comprising, in combination, a cold rolling reducing mill for reducing the diameter of a welded tube which advances continuously through the reducing mill, the latter tensioning and stretching the tubing while longitudinally advancing the same; welding means located along the path of movement of the tubing in advance of said reducing mill for welding together the edges of a slotted tube, said welding means including rolls which advance the edges of the slotted tube toward each other to form a tube of substantially circular cross section and said latter rolls being freely rotatable, said reducing mill pulling the slotted tube through and beyond said welding means; and deforming guide means located along the path of movement of the tubing between said welding means and said reducing mill for slightly deformingthe cross sectional configuration of the tubing to a non-circular configuration to prevent torsional forces created in the tubing in said cold rolling reducing mill from backing up beyond said deforming guide means to said welding means, said reducing mill pulling the tubing through said deforming guide means.

2. Apparatus as recited in claim -1 and wherein said reducing mill includes successive roll sets all except the last of which include a pair of cooperating rolls each formed with a groove of substantially semi-elliptical con figuration, the groove surfaces of the cooperating rolls forming together a substantially closed elliptical passage and engaging the tubing passing therethrough substantially along the whole outer surface thereof.

3. Apparatus as recited in claim 1 wherein said deforming guide means are constituted by guide roll means consisting of freely rotatable guide rolls.

4. Apparatus as recited in claim 3 and wherein said guide roll means includes successive roll sets which alternately deform the tithing into oval and circular cross sections.

5. Apparatus as recited in claim 4 and wherein all of the rolls of said guide roll means have parallel axes, respectively.

6. Apparatus as recited in claim 3 and wherein said reducing mill includes successive roll sets all except the last of which provide the tubing with an oval cross section, the last roll set of said reducing mill providing the tubing with a circular cross section.

7. Apparatus as recited in claim 6 and wherein the successive roll sets of the reducing mill are respectively located in planes inclined by one with respect to the next, all of said planes being inclined at an angle of 45 with respect to a vertical plane, and said welding means providing a weld located in said vertical plane so that the deformation resulting from the roll sets of said reducing mill does not act directly on the weld.

8. A method of continuously producing welded tubing of small inner diameter, comprising the steps of welding the edges of a slotted tube to each other while continuously advancing the tube by pulling forces; passing the thus welded tube continuously through consecutive roll sets of a cold rolling reducing mill while driving the rolls of the consecutive sets at respectively increasing speeds which tension the tubing in excess of its yield point while decreasing the diameter of the tubing from roll set to roll set and providing by thus driving the rolls the pulling forces advancing the slotted tube during welding; and slightly deforming the cross-sectional configuration of the Welded tubing to a cross section of non-circular configuration during the continuous movement of said tubing subsequent to the welding thereof but in advance of the reducing mill While advancing the tubing solely by the tension derived from the reducing mill for preventing torsional forces created in the tubing in the reducing mill from backing up to the tube portion being welded.

9. A method as recited in claim 8 and wherein the rolls of the consecutive sets of the reducing mill are driven at peripheral speeds increased by about /2 to 3% beyond the speeds required for the elongation of the tubing in the reducing mill.

10. A method as recited in claim 8 and wherein the successive roll sets of the reducing mill act in planes which are alternately inclined with respect to each other by 90 and which are all spaced from the line along Which the 10 edges of the tubing are welded to each other so that the deformation of the tubing by the rolls of the reducing mill never is applied directly to the weld.

11. A method as recited in claim 8 and wherein all except the last of the roll sets of the reducing mill give the tubing an oval cross section while the last roll set gives the tubing a circular cross section.

References Cited in the file of this patent UNITED STATES PATENTS 446,498 Williams Feb. 17, 1891 617,363 Skogse Ian. 10, 1899 1,352,493 Wolffgram Sept. 14, 1920 1,812,409 Leckie June 30, 1931 1,888,607 Offutt Nov. 22, 1932 1,944,096 Mayweg Jan. 16, 1934 1,984,083 Riemenschneider Dec. 11, 1934 2,017,360 Waterman Oct. 15, 1935 2,156,934 Barrett May 2, 1939 2,400,889 Ridder May 28, 1946 2,693,632 Heussner Nov. 9, 1954 FOREIGN PATENTS 759,819 Great Britain Oct. 24, 1956 

1. APPARATUS FOR CONTINUOUSLY PRODUCING SMALL SECTION WELDED TUBING COMPRISING, IN COMBINATION, A COLD ROLLING REDUCING MILL FOR REDUCING THE DIAMETER OF A WELDED TUBE WHICH ADVANCES CONTINUOUSLY THROUGH THE REDUCING MILL, THE LATTER TENSIONING AND STRETCHING THE TUBING WHILE LONGITUDINALLY ADVANCING THE SAME; WELDING MEANS LOCATED ALONG THE PATH OF MOVEMENT OF THE TUBING IN ADVANCE OF SAID REDUCING MILL FOR WELDING TOGETHER THE EDGES OF A SLOTTED TUBE, SAID WELDING MEANS INCLUDING ROLLS WHICH ADVANCE THE EDGES OF THE SLOTTED TUBE TOWARD EACH OTHER TO FORM A TUBE OF SUBSTANTIALLY CIRCULAR CROSS SECTION AND SAID LATTER ROLLS BEING FREELY ROTATABLE, SAID REDUCING MILL PULLING THE SLOTTED TUBE THROUGH AND BEYOND SAID WELDING MEANS; AND DEFORMING GUIDE MEANS LOCATED ALONG THE PATH OF MOVEMENT OF THE TUBING BETWEEN SAID WELDING MEANS AND SAID REDUCING MILL FOR SLIGHTLY DEFORMING THE CROSS SECTIONAL CONFIGURATION OF THE TUBING TO A NON-CIRCULAR CONFIGURATION TO PREVENT TORSIONAL FORCES CREATED IN THE TUBING IN SAID COLD ROLLING REDUCING MILL FROM BACKING UP BEYOND SAID DEFORMING GUIDE MEANS TO SAID WELDING MEANS, SAID REDUCING MILL PULLING THE TUBING THROUGH SAID DEFORMING GUIDE MEANS. 